1. Field of the Invention
This invention relates to an electrophotographic printer including a contact type transferring mechanism.
2. Description of Related Art
Electrophotographic printers are conventionally formed to contact a photosensitive drum carrying a toner image on its surface with a transfer roller applied with a voltage for attracting the toner and to rotate them, thereby transferring the toner image onto a print paper by inserting the print paper between the photosensitive drum and the transfer roller. In such an electrophotographic printer, the transfer roller is formed with a rubber material having the resistance around 10.sup.8 .OMEGA.-cm and covering a conductive shaft, to which a voltage of the polarity opposed to that of the toner is applied. When the print paper is inserted between the photosensitive drum and the transfer roller, a power supply for a transferring mechanism conducts a constant current control in which a constant amount of current is applied to the conductive shaft, thereby holding the voltage generated by the current. After the print paper is inserted between the photosensitive drum and the transfer roller, or during the transfer operation, the power supply then conducts a constant voltage control using the already held voltage. This control allows the electrophotographic printer to correspond to the resistive variance of the transfer roller due to environmental changes.
However, since the conductive shaft is controlled to receive the constant current when the print paper is inserted and the shaft holds the voltage generated through the constant current control, even if the resistance of the inserted print paper is changed, such a conventional electrophotographic printer can not adjust the generated voltage to the changes of the resistance. Therefore, the transfer efficiency of the electrophotographic printer may be lowered in conjunction with quality, thickness, size and the like of the print papers. For example, if the print paper has a narrow width, the resistance of the outside of the edge of the print paper becomes lower than the resistance via the paper because the photosensitive drum and the transfer roller directly contact with each other at the outside of the edge. Accordingly, even if the power supply conducts the constant voltage control by holding the voltage generated by the constant current control, current may go around the paper at the transfer roller, so that the current that should to flow through the print paper may flow into a portion of the transfer roller not contacting the print paper. Hence, in the case when the conventional electrophotographic printer prints on envelopes and the like, whose width is narrow and whose resistance is high, the transfer efficiency of the printer may be lowered.